Recently, widely spread has been a projection type image display device capable of projecting a good projection image with a large image while designed as use for a short distance. In a projection optical system of such a projection type image display device, it is already known to adopt a so-called oblique projection optical system that uses a lens having a complicated lens surface combined with a concave mirror having an eccentric aspherical shape in order to realize projection of a good projection image with a large image.
For example, Patent Document 1 discloses a projection optical device which has a projection lens comprising: a first optical system including a transmission type refractive element; and a second optical system with a reflection type refractive element, and in which some lenses of the first optical system are accommodated in a lower space regarding a lower end of the second optical system as a lower limit.
In addition, Patent Document 2 discloses a projection type image display device capable of projecting, by a combination of a lens system and a concave mirror, a good projection image with a large image while designed as use for a short distance.